1. Field of the Invention
The invention relates generally to digital communication systems and, more particularly, to systems utilizing reference symbols to assist in the demodulation of transmitted information.
2. Description of the Related Art
Digital communication systems typically utilize bandwidth-efficient modulation schemes to maintain high bit rates for a number of user channels. Such systems transmit bursts of information symbols that may include both reference symbols and data symbols. The reference symbols are known by a receiver to assist in the demodulation of the data symbols.
Some signal bursts are designed to provide control information only. For instance, FACCH (fast access control channel), SDCCH (stand-alone dedicated control channel), and SACCH (slow associated control channel) bursts provide messaging information for upper layer protocols. These and other types of signal bursts are to be distinguished from data bursts that carry voice and/or other data information, such as TCH (normal traffic channel) and RACH (random access channel) bursts. In order to benefit from these different types of bursts, the receiving portion of the communication system must be capable of distinguishing between burst types. Past systems have classified incoming bursts by devoting a portion of a preamble segment to identification data.
In order to demodulate the identification data (and the rest of the data contained in the signal burst), the clock in the receiver must be synchronized with the clock in the transmitter and, further, the oscillator in the receiver must be aligned with the actual frequency of the carrier signal. With certain modulation schemes (e.g., QPSK), the receiver is capable of deriving timing and frequency information from the information symbols themselves subsequent to removing the modulation. These conveniences are not available with modulation schemes such as GMSK (Gaussian Minimum Shift Keying), in which modulated information is transmitted over a plurality of bit periods, inasmuch as no single non-linearity exists for modulation removal. However, such memory-inducing modulation schemes provide several advantages, not the least of which is the constant envelope of the transmitted signal. This advantage is particularly useful for satellite communication systems, inasmuch as cheaper Class C amplifiers may be used in both the satellites and the receivers.
In past systems utilizing memory-inducing modulation schemes, the preamble segment of each signal burst also included portions for estimating the timing and frequency of the signal burst. For the timing variable, a first portion provided reference symbols having considerable variation such that the transmitted signal has high frequency components. A second portion provided symbols all set to "1" to obtain high resolution for the frequency variable. However, evaluation of these portions of the preamble became an undesirable prerequisite to burst type classification. Moreover, allocating separate bits to timing and frequency estimation as well as burst type classification has limited the data throughput of such systems.